qapi script: do not allow string discriminator

Since enum based discriminators provide better type-safety and
ensure that future qapi additions do not forget to adjust dependent
unions, forbid using string as discriminator from now on.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

tests/test-qmp-input-strict.c

@@ -146,7 +146,10 @@ static void test_validate_union_flat(TestInputVisitorData *data,
     Visitor *v;
     Error *errp = NULL;
 
-    v = validate_test_init(data, "{ 'string': 'a', 'boolean': true }");
+    v = validate_test_init(data,
+                           "{ 'enum1': 'value1', "
+                           "'string': 'str', "
+                           "'boolean': true }");
     /* TODO when generator bug is fixed, add 'integer': 41 */
 
     visit_type_UserDefFlatUnion(v, &tmp, NULL, &errp);